Abstract. -A new approach explaining the main forcing factor of Hirnantian glaciation is proposed herein. It follows the models associating occurrences of continental glaciations with periods of low atmospheric CO 2 levels. The accumulation of great volumes of carbonates during pre-Hirnantian late Ordovician, in regions where these deposits were previously absent, is suggested as a major sink of atmospheric CO 2 . This would have caused an important lowering of the average temperature in the early Hirnantian, after CO 2 values had attained a certain threshold. This process was maintained by other positive feedbacks, such as the short-term carbonate weathering CO 2 sink. An increase of the direct flux of CO 2 from the atmosphere to the oceans by means of dissolution would have been driven by the enhancement of carbonate deposition. The great inundation of the low latitude Laurentia craton during Cincinnatian times and the establishment of a temperate-water carbonate sedimentation on the North Gondwana margin during pre-Hirnantian Ashgill allowed the burying of more than 840 × 10 15 kg (1.9 × 10 19 mol) of dissolved CO 2 . This mass is equivalent to nearly 350 times the present values of atmospheric CO 2 . This is important enough to have greatly altered the equilibrium between the CO 2 dissolved in the oceans and the partial pressure of CO 2 in the air, eventually causing an important reduction of the latter. The new model also offers a simple explanation for the end of the glaciation after a short time-span. Glacioeustatic lowering of the sea level, concomitant with the glaciation, would have stopped the extra-sedimentation of carbonate due to the retreat of the oceans from the platforms, closing this CO 2 sink. Pre-glacial CO 2 levels would then recover, due to volcanic and metamorphic CO 2 outgassing. After subsequent melting of the ice cap, oceanic circulation did not recover pre-Hirnantian Ashgill strength, resulting in a strong stratification of ocean waters and precluding the recovery of an extensive carbonate deposition. The well-known positive shift in the δ 13 C at the base of the Hirnantian is assumed to have been caused by weathering and dissolution of carbonates, relatively enriched in 13 C, during the glacioeustatic regression and exposure of the platforms.La sédimentation carbonatée ordovicienne : un des principaux facteurs déclencheur de la glaciation hirnantienneMots clés. -Carbonates, Glaciation, Puits de CO 2 , Hirnantien, Gondwana septentrional.Résumé. -Une nouvelle approche concernant le déroulement de la glaciation hirnantienne est proposée dans ce travail. Elle s'intéresse aux principaux facteurs clés de cette dernière et associe les effets d'une glaciation continentale à une pé-riode de bas niveau du CO 2 atmosphérique. L'accumulation d'un important volume de carbonates au cours de l'Ordovicien terminal pré-Hirnantien dans des régions où ces derniers étaient antérieurement absents est considérée comme un important puits de CO 2 atmosphérique. Cette accumulation pourrait être la cause d'une...

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The Angosto de Lampazar, a classic locality for the study of lower Paleozoic successions in the Cordillera Oriental, NW Argentina, has yielded a late Cambrian relatively diverse, lingulate brachiopod fauna. Sandy lenses with calcareous cement from the uppermost levels of the Lampazar Formation have yielded abundant remains of articulate and phosphatic brachiopods. Among the latter, the new speciesEurytreta harringtoniMergl and Herrera,Lingulella?melonicaMergl and Herrera,Libecoviella lenticularisMergl and Herrera, andSchizambon cardonalisMergl and Herrera, as well as the new genus and speciesSaltaia lampazarensisMergl and Herrera are formally introduced. Trilobites and conodonts from the same horizons characterize theCordylodus proavusZone, allowing a correlation with Stage 10 of the Furongian Series. Although the information on lingulate brachiopods from theC.proavusZone is scarce across the world, the composition of the studied association displays a relationship with coeval and slightly younger faunas of Utah and Kazakhstan. The Argentine brachiopods, the first described from the late Cambrian Proto-Andean margin of Gondwana, are more closely related to temperate Laurentian faunas than to those from the high latitude North African margin of Gondwana.

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